Project Summary/Abstract Middle East Respiratory Syndrome (MERS) was recognized as a significant illness on the Saudi Arabian peninsula in mid-2012, and the causative agent was rapidly identified as a novel coronavirus (CoV), termed MERS-CoV. MERS has a high mortality (~35%), associated with severe lung disease. Similar to the SARS virus that caused an epidemic in 2003-4, there is ongoing global concern due to MERS high fatality rate. To date, cases of MERS have been reported in 26 countries. Dipeptidyl peptidase 4 (DPP4, CD26) is the receptor for MERS-CoV. Epidemiologic studies have established that MERS is zoonotic in origin, with evidence for a closely related virus in dromedary camels on the Arabian peninsula and throughout Africa. Spread from camels to people is documented, as well as person-to-person spread among health care workers in hospital settings. A lack of autopsy studies from MERS fatalities has hindered understanding of MERS-CoV pathogenesis. Thus, MERS is the most recent confirmation that coronaviruses can jump from their animal hosts, infect humans, and cause severe disease of global significance. There is a pressing need to better understand MERS disease pathogenesis and to develop vaccines and therapies. There are 3 specific aims. Aim 1. To understand how an in vivo evolved MERS-CoV causes lethal lung disease. We developed mice that have the human receptor for MERS-CoV. Using these animals we developed a mouse-adapted virus that causes significant lung disease. These studies will advance our knowledge of the causes of MERS-related lung disease. Aim 2. To investigate how adaptive mutations in MERS-CoV contribute to increased virulence. We sequenced the mouse-adapted virus strains and assembled their genomes. We will use this genetic information to investigate relationships between the virus gene products and the host responses that lead to severe lung disease. Aim 3. To investigate how DPP4 abundance and function influence MERS disease pathogenesis. DPP4 has enzymatic activity that cleaves two amino acids off of target protein substrates, thereby changing protein functions. DPP4 abundance and enzymatic activity may contribute to disease. These experiments will advance our knowledge of how DPP4 activities may underlie to disease outcomes.